The noisy environment in our industrial society is a health hazard to numerous workers as well as to people engaged in recreational activities generating loud noises.
Users often wear earplugs when operating light machinery such as chainsaws or heavy machinery such as paper industry, printing industry, aircraft industry machines, when participating in sporting activities such as shooting, and when attending various spectator events such as automobile races, truck pulls, and rock concerts, and the like.
The art generally refers to different types of earplugs such as “universal fit” type earplugs that are intended to adapt to the contours of any person's ear canal to provide hearing protection; custom-molded earplugs that have advantages in their comfort, more reliable fit and lower long-term costs due to longer usable life; and semi-custom-molded expandable earplugs that are pre-shaped earplugs having the approximate shape of the ear canal of the individual and expanded with a settable compound material injected therein.
All the above specifically refer to earplugs but it is to be understood that it is similarly applicable to any in-ear device, the latter referring to either earplug device (or hearing protection device (HPD)) or hearing aid device (HAD) for which an attenuation level or an amplification performance level is seek respectively.
One important aspect of preventing hearing loss is the accurate determination of protection from noise offered by an HPD. Protection must be sufficient to protect hearing from noise damage, but should not over-attenuate and interfere with communication and warning signal detection.
Current HPD evaluation is done by using a variety of technical evaluations, statistical corrections and estimations.
Real-Ear Attenuation at Threshold (REAT) is a subjective method of determining the attenuation of a hearing protection device by subtracting the open-ear (unprotected) threshold of hearing from the occluded ear threshold (with the hearing protector in place).
The method for determining REAT is similar to the standard hearing test. The subject is tested in the following manner. Specific tones are given and subject responds when the tones are heard. The hearing threshold is determined based on a given number of positive responses at given sound levels. The REAT will therefore represent the value of attenuation of the HPD reported by the tested individual.
Noise Reduction Rating (NRR) is an estimate of hearing protection capability determined by applying a statistical analysis to a series of REAT measurements. It is a single value figure that estimates the minimum noise reduction measurement theoretically obtained by 98% of the individuals in a laboratory setting.
This percentile of 98% is obtained by subtracting, for every octave band, twice the standard deviation from the mean attenuation measurements reported during the REAT test done according to ANSI 53.19. This is what the American National Standard requirests for NRR calculation by the US Environmental Protection Agency (EPA).
The Personal Attenuation Rating (PAR) is a single number value that represents the individual attenuation that each laboratory subject obtained in the REAT test: it is indeed equivalent to a “personal NRR”. For example, the thirty PAR values obtained during an ANSI S3.19 test on an expandable type in-ear device as disclosed in U.S. Pat. No. 6,687,377 to Voix et al. granted on Feb. 3, 2004 were recorded.
For this certification test, PAR values range from 18 dB (obtained in one trial) to 34 dB (obtained in two trials). The NRR calculated from this test series, due to the subtraction of two standard deviations, is 15 dB. This is consistent with the very conservative NRR approach of estimating protection for 98% of users, but is virtually useless in determining individual protected values.
Additionally, there is no objective way of measuring an insertion loss (IL) value provided by an in-ear device. The IL estimation described in all standards (ANSI, ISO, CSA, etc.) is subjectively determined by the individual wearing the in-ear device, as better described hereinbelow.
All standards, such as ANSI, ISO, CSA and the like, require an insertion loss (IL) subjective estimation, generally expressed in dB (decibels), of the acoustic seal provided by the in-ear device based on a ratio of REAT values determined at the tympanic membrane, or eardrum, by the individual himself (thereby subjective), with and without the in-ear device.
Examples of assessments of acoustical performance of in-ear devices are found in the following documents:                U.S. Pat. No. 5,970,795 granted to Seidmann et al. on Oct. 26, 1999 for “Apparatus and method for testing attenuation of in-use insert hearing protectors”;        U.S. Pat. No. 5,757,930 granted to Seidmann et al. on May 26, 1998 for “Apparatus and method for testing attenuation of in-use insert hearing protectors”;        U.S. Pat. No. 5,577,511 granted to Killion on Nov. 26, 1996 for “Occlusion meter and associated method for measuring the occlusion of an occluding object in the ear canal of a subject”;        U.S. Pat. No. 5,317,273 granted to Hanson et al. on May 31, 1994 for “Hearing protection device evaluation apparatus”; and        U.S. Pat. No. 4,060,701 granted to Epley on Nov. 29, 1977 for “Method for testing acoustical attenuation of hearing protectors”.        The last method taught by Epley is another subjective evaluation method and suffers from the same weaknesses as all the other subjective methods, naming:        the subjectivity of the measurements is a great source of uncertainty and also significantly reduces the possibility of repeatability of the measurements.        the subjective estimation of the attenuation is always larger than the objective measurement of the corresponding IL, especially in the low-frequencies; the “Occlusion Effect” tends to increase the physiological noise (PN) present behind the protector by modifying the acoustic radiation impedance seen from the tympanic membrane.        
Other ways of measuring acoustical attenuation or acoustic seal of an in-ear device disclose some devices that could measure the pneumatic pressure leakage of an in ear-device to later on predict its acoustical attenuation or the presence of an “acoustic seal”. Obviously, this mere static pressure drop measurement is insufficient to reliably predict the acoustic pressure drop, and numerous materials may prove to provide excellent pressure seal and still perfectly have sound pressure transmitted there through. For example, a ping-pong shell molded in the ear could be tightly sealed therein, but will always transmit sound there through.
Accordingly, there is a need for an apparatus and method for objective assessment of in-ear device acoustical performance.